Recent inquiry into variation and evolution of reproductive systems of flowering plants has produced exciting and ground-breaking work within the plant sciences. In a special issue of the International Journal of Plant Sciences, leading botanist Spencer Barrett of the University of Toronto gathers together in a single volume some of the most compelling work on the three areas that have seen the most growth: flowers and pollination, mating patterns and gender strategies, and asexual reproduction and polyploidy. The focus of the volume is on evolutionary transitions. The shifts in reproductive character states that constitute evolutionary transition are key elements of biological diversification and the identification and study of major transitions during the history of plant life now represent an important research program in evolutionary biology. This volume is the first to address the study of evolutionary transitions in plants.

Flowering plants owe their spectacular diversity to the evolution of their pollination systems, and the first section of the issue, Flowers and Pollination, includes five articles that explore floral function. Mark Raushers article begins the special issue with a comprehensive review of transitions in flower color in animal-pollinated lineages, focusing in particular on the change from blue to red. James Thomson and Paul Wilson follow with a paper that also studies the blue-to-red change, but they consider the ecological and genetic mechanisms that might account for the destabilization of pollination syndromes. Risa Sargent and Jana Vamosi further explore this topic in their paper, which investigates the extent to which ecological context influences evolutionary transitions in the degree of pollinator specialization. Next, Lawrence Harder and Steven Johnson tackle the intriguing problem of why some flowering plants disperse their pollen in groups. And in the final article in the section, William Friedman, Eric Madrid, and Joseph Williams propose a novel evolutionary and developmental perspective on the structural diversity of female gametophytes in angiosperms.

In the first article of the section Mating Patterns and Gender Strategies, Boris Igic, Russell Lande, and Joshua Kohn examine the breakdown of self-incompatibilitythe principal and most effective mechanism preventing self-fertilizationand its evolutionary consequences. In the second article, Stephen Wright, Rob Ness, John Paul Foxe, and Spencer Barrett pick up on the theme and review the genomic consequences of selfing and outcrossing and preview future developments in the field, as the wealth of new data becomes available. Daniel Schoen and Jeremiah Busch use a novel metapopulation perspective in their discussion of the importance of group-level selection of mating systems. The final two articles in the sectionthe first by John Pannell, Marcel Dorken, Benoit Pujol, and Regina Berjano; the second by Andrea Case, Sean Graham, Terence Macfarlane, and Spencer Barrettconsider transitions in gender strategies in specific groups with exceptional sexual system diversity.

In the final section, Asexual Reproduction and Polypoidy, Jonathan Silverton begins by evaluating the costs and benefits of asexual and sexual reproduction in plant species that have both reproductive modes. His article is followed by Jeannette Whitton, Christopher Sears, Eric Baack, and Sarah Otto's comprehensive review of the evolution of apomixis, or asexual reproduction without fertilization. Next, Stacey Lee Thompson, Gina Choe, Kermit Ritland, and Jeanette Whitton employ new approaches to investigate the presence and extent of asexuality and recombination within populations of the Easter Daisy (Townsendia hookeri). In the final article, Brian Husband, Barbara Ozimec, Sara Martin, and Lisa Pollock synthesize two themes from earlier articlespolyploidy and mating systems. In the course of their study, they find that the costs of selfing in polyploids are likely to be dynamic, changing with the age of the polyploid and history of mating.

As a collection, this special issue of the International Journal of Plant Sciences forms an essential volume for anyone interested in plant evolutionary biology.

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